Hours after Hillary Clinton stumbled as she departed a Sept. 11 memorial service, her campaign announced she’d been diagnosed with pneumonia. Later that evening, the candidate received a second opinion. “I must advice the Clinton campaign to perform toxicologic analysis of Ms. Clinton’s blood,” Dr. Bennet Omalu wrote on Twitter. “It is possible she is being poisoned.”

This was just the latest from a man who has made a habit of delivering bold—one could also say “unfounded” or “irresponsible”—medical assessments to a story-hungry, star-struck press. In January, he declared that O.J. Simpson has chronic traumatic encephalopathy, despite the fact that the football-induced brain disease can only be diagnosed via postmortem dissection. “I would bet my medical license that he has CTE,” Omalu told People. In an email exchange this week, Omalu stood by this wager, adding that most violent prisoners in the United States—between 60 and 100 percent, he says—have a history of traumatic brain injuries.

Omalu stands by his Clinton tweets as well. When he claimed the candidate might have been poisoned, he told me, he meant it in the sense that any drug can be poisonous—including prescription drugs and alcohol. “The media misconstrued it and sensationalized it to suggest or insinuate that Ms. Clinton was intentionally being poisoned by someone else,” Omalu said. As for the reference to Trump and Putin, he pointed out he’s “a student of literature” inclined to dabble in satire and hyperbole.

In Concussion, Omalu is depicted as a truth-teller, a man of science and of faith who bravely took on the corporate giants of the NFL, arming himself only with the facts. But his bouts of overconfidence and his freewheeling public statements have gotten much less play: We don’t want to think that the David in this David-versus-Goliath tale is now too enamored of his sling. Omalu found his fame by stubbornly pursuing what at the time appeared to be a far-fetched intuition. He dared to push a doozy of a theory—that playing football can corrode a person’s brain—with only modest data to support it, and he turned out to be correct. His impulse to keep on pursuing his unusual beliefs helped make him into a hero, one bestowed with well-deservedhosannas. In the years since, though, that same quality has at times led him astray.

As journalist Jeanne Marie Laskas made clear in her book Concussion, Omalu has himself transformed from a Nigerian immigrant with $250 in his pocket to a famous scientist—an accomplished man of distinction. He’s been quick to lash out at the lesser minds who have tried to undercut his work. When NFL-backed researchers attacked his original reports on CTE, he says he quickly realized these other scientists were outgunned intellectually. “Their understanding of the subject was embarrassingly naïve and virginal,” he wrote.

It hasn’t helped Omalu’s reputation that he loves to talk about the spirits—and also to the spirits—of the men whose brains he slices up. These ghosts once gave him a flat tire, he says; they even turned on his dishwasher. “I practice my faith in my science, and my science in my faith,” he told me via email. When Omalu first examined the body of Mike Webster, a Hall of Fame center for the Pittsburgh Steelers who had suffered a dramatic cognitive decline and then died from four blocked arteries in 2002, he touched the dead man’s forehead and asked for guidance. “Mike, you have been misjudged,” he told the spirit. “We have to prove them wrong.”

At the time, Omalu was working at the Allegheny County Coroner’s Office. He knew little about football except that it appeared to be quite violent and that “the players ran into one another a whole lot and banged their heads repeatedly like guinea pigs.” According to Laskas’ book, he didn’t even realize the corpse he’d been assigned—the one that would provide the first important evidence of CTE in former players—belonged to someone famous. Omalu soon remembered he’d seen a piece about Webster on CNN. The news had made him angry; he could tell this football player had not been treated fairly: “I thought he was rather a victim of the game and not of himself.”

Before he started on his autopsy, Omalu knew exactly what he’d find—that Webster had succumbed not to cardiovascular disease but to all those years of getting knocked around. Now Omalu only had to prove his intuition in the lab. When he opened Webster’s skull, he expected to find a shrunken, damaged brain. Alas, he found no sign of injury. “Honestly, I was extremely disappointed when I looked at Mike Webster’s brain,” Omalu wrote. “I may have said in my heart to Mike, hey Mike, what is going on? Please do not let me down.”

If Omalu had looked more deeply into Webster’s life, he might have had less confidence in his out-there diagnosis. The former Steeler had a horrifying past, with risk factors for mental illness that ran through several generations of his family, and that started long before he joined the NFL. Both his parents had been alcoholics, and he’d been abused. All his siblings were bipolar, and his uncle had committed suicide. Another sort of doctor might have blamed Webster’s tragic death on this history of off-field trauma.

Another sort of doctor, someone less stubborn and ambitious, would have let the matter go. But Bennet Omalu stuck to his belief in spite of countervailing evidence. He preserved and sliced the brain, then sent it out for staining, and the specimens confirmed his hunch. Under the microscope, he found a buildup of tau proteins, the hallmark of degenerative disease.

We should all be thankful for Omalu’s finding, which did a tremendous amount to increase awareness of the risks of head injuries in football. In the years that followed, though, his confidence expressed itself in less useful ways. He soon became convinced, for example, that 90 to 100 percent of football players have CTE and that the disease can lead to suicide. (Omalu has a long-standing interest in suicides, and what can be done to prevent them. “We should become more family aggressive,” he once explained. “We should stop divorcing.”) In 2007, the New York Times reported his opinion that the suicide of former safety Andre Waters was “almost certainly exacerbated, if not caused, by the state of his brain.” In Omalu’s self-published book Play Hard, Die Young, out the following year, he guessed that about one in five football players will “develop major depression and possibly suicidal behavior.” He now lists suicidal behavior, suicidal thoughts, attempted suicides, and completed suicides among the constellation of symptoms that make up CTE.

That idea, widely promulgated in the press, seems overblown. The effects of repeated head injuries on athletes (e.g. “punch drunk syndrome”) have been described in the medical literature since the 1920s, but suicide was rarely mentioned before the last few years. In fact, former football players seem to commit suicide at far lower rates than other people despite their higher prevalence of risk factors such as chronic pain. In a recent update to a study of more than 3,000 NFL retirees, researchers at the National Institute of Occupational Health and Safety found that 12 men in the group had killed themselves—less than half as many as would be expected in a group of race- and age-matched controls. Another recent study looked at 477 deaths among college athletes between 2003 and 2012. Football players appeared to be somewhat more likely to commit suicide than other athletes, but they were still at much lower risk than other male college students.

Unsupported ideas about head trauma may themselves be dangerous. Last December, a week before Concussion premiered in theaters, a 24-year-old former football player named Zac Easter shot himself in the heart. He had become convinced he was suffering from CTE—a disease he understood to be incurable, progressive, and devastating. In his suicide note he asked that his brain be sent to Omalu, who ultimately confirmed the presence of tau-protein pathology. It’s impossible to know whether Easter’s media-fueled self-diagnosis or his underlying brain condition was the more important factor leading him to take his life.

Omalu has been quick to make diagnoses of CTE. He’s made them over the phone, telling the wife of one player, “You know what? I could bet my life your husband has CTE.” (When that player died, Omalu turned out to be correct.) “As a forensic pathologist, I am also trained to observe not just dead people, but living people as well,” he told me, and to form opinions based on the available evidence. He’s also said it’s possible to suss out the disease in living players using radioactive tracers and brain scans. Omalu has a financial stake in a company he founded, TauMark, that holds a patent on that method, and he’s been vocal about its promise. When combined with clinical evaluations of a player’s symptoms, he has said, one could diagnose a case of CTE with “a reasonable degree of certainty.”

It’s way too early to make that claim. The tracer that TauMark uses is not specific to tau proteins and CTE. It also seems to pick up beta-amyloid—a sign of Alzheimer’s—as well as the misfolded proteins that are associated with Creutzfeldt-Jakob disease. In February, the Food and Drug Administration sent a warning letter to Omalu’s business partner Gary Small, a neurologist and neuro-self-help author, admonishing TauMark for making unfounded assertions about the safety and efficacy of its test. “We took note of the FDA’s recommendations and implemented all of them,” Omalu said in his email. He’s now working on a case study of former Minnesota Vikings linebacker Fred McNeill, a test subject for TauMark’s technology who died in November. “It will be an epoch-making paper, just like the Mike Webster paper,” the doctor told me.

Omalu can exaggerate in other contexts, too. In 2009, for example, the doctor offered expert testimony in a lawsuit against Dow Chemical, arguing that a Pennsylvania exterminator had contracted non-Hodgkin’s lymphoma from one of the company’s insecticides. But a judge threw out his report, citing the lack of evidence to support its claims. According to an appeals court ruling, the court had “rightly concluded that Dr. Omalu’s proposed testimony was unreliable due to numerous cracks in its scientific foundation.” (To be clear, this judge’s rebuke of Omalu was an outlier. He has succeeded in testifying as an expert witness in hundreds of cases.)